The invention relates to a method and an arrangement for defining transmission power in a mobile station. The invention can preferably be applied in a TDMA (Time Division Multiple Access) cellular system which offers broadband services and in which the number of time slots used for data transfer can be changed during the connection.
In present-day mobile station networks, a so-called time division multiplexing, i.e. TDMA (Time Division Multiple Access) method is generally used. For example, in the GSM (Global System for Mobile communications) system, TDMA frames which consist of eight time slots are used on each frequency channel intended for communications use. Traditionally in mobile communication systems connection is established so that one time slot is allocated for the connection and the data transfer channel thus formed is often used during the entire connection. If the mobile station, however, is translocated from the service area of one base station to the service area of another base station, the change of an active base station, i.e. a so-called handover takes place, and then a channel using a new time slot is formed between the new active base station and the mobile station. One can transfer to a new channel also in such a case that one wishes to rearrange the channels used by the base station, for example, to minimize interference. The implementation of the new channel is not then connected to the change of the active base station.
FIG. 1 shows a TDMA frame of the GSM system which comprises eight time slots 0-7. In the figure, a reception frame RX and a transmission frame TX have been shown separately. The reception frame means herein the frame received by a mobile station, i.e. a TDMA frame of downlink transfer direction. The transmission frame means herein the frame transmitted by a mobile station, i.e. a TDMA frame of uplink transfer direction. A time slot 1 has been marked with an X in FIG. 1 and it is used in data transfer of both uplink and downlink transfer direction in the connection described by the example.
FIG. 2 shows the use of time slots in the connection according to FIG. 1 after a new time slot 3, which has been implemented instead of time slot 1, has been allocated for the connection. This new time slot can be in the same TDMA frame as the old time slot used by the connection, or it can also be a TDMA frame which is on a different frequency.
Especially for data transfer services, broadband HSCSD (High Speed Circuit Switched Data) services have been implemented in which the connection uses more than one time slot for speeding up data transfer. Then the numbers of time slots to be used can be equal in uplink and downlink transfer directions, in the case of a symmetric configuration, or they can be unequal, which is termed an asymmetric configuration. The time slots in use are defined at call establishment when the system informs the mobile station which time slots are to be used and the parameters connected to them, such as the transmission power to be used at each time slot.
The number of time slots in use can be changed also during the connection. The change occurs in accordance with the initiative from the system so that the system transmits to the mobile station a CONFIGURATION CHANGE COMMAND message which contains a proposal for a new time slot configuration. The mobile station responds to this with a CONFIGURATION ACKNOWLEDGE acknowledgement. The aforementioned message is just an alternative by means of which the number of time slots in an HSCSD connection can be changed. The said alternative is however preferable because it requires only a small amount of signalling.
FIG. 3 shows a TDMA frame in a connection as an example in which two time slots 0 and 1 are used in a downlink transfer direction RX and one time slot 1 in an uplink transfer direction TX. FIG. 4 shows the use of time slots after the reconfiguration when one new time slot 2 has been implemented for the connection in both downlink and uplink transfer directions.
When a new time slot is allocated for the connection, the mobile communication system calculates the optimal transmission power for the new time slot for both the downlink and the uplink transfer direction. The value of transmission power calculated for the uplink transfer direction is transferred to the mobile station, for example, on a signalling channel. Said CONFIGURATION CHANGE COMMAND message does not contain information, for example, about the value of the transmission power to be used on the new time slot.
Fundamental disadvantages are, however, associated with the solutions according to prior art. In known mobile phone systems, e.g. in the GSM system, power setting parameters are transmitted at communications SACCH (Slow Associated Control CHannel) time slots at intervals of 0.5 seconds. If, at the implementation of a new time slot, the transmission power is set, for example, at its maximum value, it stays at this value perhaps for as long as 0.5 seconds before the power setting parameter is received from the base station.
When the power setting parameter has been received, the mobile station changes the value of the transmission power according to the parameter concerned step by step. In the GSM system, there are 15 steps between the minimum and maximum transmission powers and it takes 60 ms to perform one power changing step. Accordingly, it takes altogether 0.9 s to change power from its maximum value to the minimum value. When the delay elapsed for the reception of the parameter, the aforementioned 0.5 s at its maximum, is added to this time, it takes altogether 1.4 s at maximum to attain the correct transmission power, and during all of this time the transmission power is too high in the example described above.
This strong transmission power which lasts for quite a long time may cause co-channel interferences to other radio links. The use of high transmission power levels reduces significantly data transfer capacity, since in this case those base stations which are using the same frequency channel must be situated at great distances from each other. Furthermore, unnecessarily high transmission power levels increase current consumption of a mobile station. In particular high transient power levels put major stresses on the power supply circuits and the RF power amplifier.
Excessive transmission power could be avoided by setting the transmission power of a new time slot at its minimum value from which power it could be elevated when necessary after the transmission power parameter has been received. A problem with this procedure is, however, that when transmission power is too low, then data transfer may fail.
One solution would be to transmit power setting parameters considerably more frequently. A disadvantage of this solution is, however, that continuous transfer of control messages on communications channels would burden significantly the data transfer capacity of the mobile communication system. Another solution would be to include the power adjustment parameter in the message which is used for informing about the implementation of a new time slot. A problem with this solution is, however, that it is difficult to arrange the space required by the parameter into the message concerned and secondly, at this stage the base station has not necessarily managed to define the correct transmission power parameter for the mobile station.
The aim of the present invention is to avoid the above described disadvantages connected to prior art by devising a solution in which the transmission power of a new time slot can immediately be made close to the optimal value without increasing signalling between the base station and the mobile station.
One fundamental idea of the invention is that the transmission power of a new time slot is defined in the mobile station on the basis of transmission power of at least one other time slot used in the connection. If there has been one time slot for the use of the connection when a new time slot is being allocated, preferably the same value is set for the transmission power of the new time slot as it had at the time slot previously in use. If there have been two or more time slots for the use of the connection when a new time slot is being allocated, the value of the transmission power of the new time slot is preferably defined on the basis of the transmission powers of these other time slots according to a previously defined algorithm. As the algorithm, a minimum value, a maximum value or an average value can preferably be used.
It is possible to use the solution according to the invention when one or more new time slots are needed for a connection in addition to the time slots which are already in the connection use. Furthermore, the method according to the invention can be applied when it is desired to transfer the connection from the old time slot (old time slots) to a new time slot (new time slots) by performing the change of channel without changing the frequency. The invention can be applied in the changes of multichannel configurations independent of signalling messages used, if the frequency used, i.e. the physical channel does not change.
The invention is partially based on the observation that on the channels between the mobile station and the base station, the values of the optimal transmission powers are very close to each other. Thus a fundamental advantage of the invention is that the transmission power of the new time slot in the mobile station can be made very close to the optimal value immediately at the implementation of a new time slot. Thus problems of interference caused by excessive transmission power can be avoided, as well as increase in power consumption and transient peaks in power consumption. Similarly, the risk that the transmission power of the new time slot would be too low to permit a successful data transfer is minimized.
Furthermore, an advantage of the invention is that, at the implementation of a new time slot, no extra data transfer is needed, and in that way the use of the method according to the invention does not burden the data transfer capacity of the connection.
A method according to the invention for defining the transmission power of a mobile station during the connection between a mobile station and a mobile communication system, when data transfer in an uplink transfer direction occurs at least at one time slot of a TDMA frame and during the connection a new time slot is allocated for the uplink transfer direction of the connection, is characterized in that the transmission power of said new time slot is defined on the basis of the transmission power of some other time slot being used in the uplink transfer direction of said connection.
A mobile station linked to a mobile communication system according to the invention, such that the mobile station comprises means for transferring data at least at one time slot of a TDMA frame and means for allocating a new time slot during the connection, is characterized in that the mobile station comprises means for defining the transmission power of said new time slot on the basis of the transmission power of some other time slot in use in the data transfer of said connection.
Preferable embodiments of the invention have been presented in dependent claims.